HD 49933: A laboratory for magnetic activity cycles
T. Ceillier, J. Ballot, R.A. Garcia, G.R. Davies, S. Mathur, T.S. Metcalfe, D. Salabert
aa r X i v : . [ a s t r o - ph . S R ] O c t Transiting planets, vibrating stars & their connectionProceedings of the 2 nd CoRoT symposium (14 - 17 June 2011, Marseille)Edited by A. Baglin, M. Deleuil, E. Michel & C. Moutou
HD 49933: A laboratory for magnetic activity cycles
T. Ceillier ∗ , , J. Ballot , , R.A. Garc´ıa , G.R. Davies , S. Mathur , T.S. Metcalfe ,D. Salabert ∗ Affiliations are listed at the end of the paper
Abstract.
Seismic analyses of the CoRoT target HD 49933 have revealed a magneticcycle. Further insight reveals that frequency shifts of oscillation modes vary as a functionof frequency, following a similar pattern to that found in the Sun. In this preliminary work,we use seismic constraint to compute structure models of HD 49933 with the AsteroseismicModeling Portal (AMP) and the CESAM code. We use these models to study the effectsof sound-speed perturbations in near surface layers on p-mode frequencies.
1. Introduction and observations
Dynamo processes in the outer convective envelope of solar-like stars generate magnetic fields,which create active regions and spots at the stellar surface (e.g. Lanza 2010). In the Sun,such a dynamo leads to a well known almost regular 22-year magnetic cycle. This dynamois far from being completely understood, and is difficult to predict as suggested by the recentunusually long solar minimum of cycle 23 (e.g. Salabert et al. 2009). Study of magnetic cyclesin other stars should allow us to better understand the physical mechanisms by studying manystars in different evolutionary stages and conditions (e.g. Chaplin et al. 2007; Metcalfe et al.2007). Variations of magnetic fields modify the stellar structure and therefore frequencies andamplitudes of the acoustic modes (Jim´enez-Reyes et al. 2003). Such variations are detectablewith current asteroseismological facilities.HD 49933 is a F5V star observed by CoRoT for 60 and 137 days in 2007 and 2008 (Ap-pourchaux et al. 2008; Benomar et al. 2009). The light curve presents clear signatures of activeregions crossing the visible stellar disk that reveal a rotation period of 3.5 days with differentialrotation. The analysis of short time series reveals frequency shifts and amplitude variations inacoustic modes unveiling short-term variations of activity, compatible with a short stellar cycleof around 150 days (Garc´ıa et al. 2010). Moreover, the frequency shifts measured in HD 49933present a clear dependence with increasing frequency, reaching a maximum shift of about 2 µ Hzaround 2100 µ Hz (Salabert et al. 2011). Such a dependence is comparable to the one observedin the Sun, which is understood to arise from changes in the outer layers due to its magneticactivity.
2. Modelling and Discussion
In this work we study the internal structure of HD 49933 paying special attention to propertiesof the external convection zone. We use the seismic and spectroscopic observations to find abest model with AMP (Metcalfe et al. 2009). We use this model as a starting point to computetwo structure models with the CESAM stellar evolution code (Morel 1997). The first modeluses the mixing-length theory (MLT, B¨ohm-Vitense 1958) for treating convection whereas thesecond use the CGM prescription (Canuto et al 1996). To quantify the sensitivity of modes tothe superficial structure, we compute upper turning points of the p modes with frequencies ν ∈ [1400–2600] µ Hz. Turning points r o are computed as 2 πν = ω c ( r o ), where ω c = c s / (2 H ) is theisothermal cut-off frequency ( c s is the sound speed and H the pressure scale height). In Fig. 1a,we see similar results for both models. Nevertheless, in the CGM model, r o depends less on ν Ceillier et al.in the range 1400–2600 µ Hz, because the temperature profile obtained with this prescription issharper than the one obtained with MLT.We crudely estimate the impact on frequencies of a perturbation in pressure due to a changein magnetic field. We assume that c s is perturbed by a change δp in pressure, all other quantitiesstaying unchanged. Thus, the travel time of waves is modified by δτ = R r o δp/ ( pc s ) dr. We thenconsider the effect on frequencies is δν ≈ ν δτ . We consider two profiles for δp : (A) δp is constantin the surface layers; (B) δp linearly grows during the first 10 Mm beneath the surface. Resultsare plotted in Fig. 1b. We use δp = 400 dyn cm − at the photosphere to recover reasonablevalues of δν . ν c [ µ H z ] −200 0 200 400 600acoustic depth [s]−1 0 1 2 4 6 8 10depth [Mm] 1000 1500 2000 2500 3000Frequency [ µ Hz]0.00.51.01.52.02.53.0 ν δ τ [ µ H z ] Figure 1.: a)
Acoustic cut-off frequency as a function of depth: MLT (solid black line) and CGM(blue dashed line) models of – b) Effects on frequencies of a pressure change following the profileA (solid lines) or B (dotted lines). Same color code as before.
Acknowledgements.
The CoRoT space mission has been developed and is operated by CNES, with con-tributions from Austria, Belgium, Brazil, ESA (RSSD and Science Program), Germany and Spain. JB,RAG, and DS acknowledge the support given by the French PNPS program and CNES. NCAR is sup-ported by the National Science Foundation.Affiliations: Laboratoire AIM, CEA/DSM-CNRS-Universit´e Paris Diderot; IRFU/SAp, Centre deSaclay, 91191 Gif-sur-Yvette Cedex, France. CNRS, IRAP, 14 avenue Edouard Belin, 31400 Toulouse,France. Universit´e de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France. High Altitude Observa-tory, 3080 Center Green Drive, Boulder, CO, 80302 USA. Universit´e de Nice Sophia-Antipolis, CNRS,Observatoire de la Cˆote d’Azur, UMR 6202, BP 4229, 06304 Nice Cedex 4, France